SYNTHESIS, CRYSTAL-STRUCTURE AND PROPERTIES OF HG2BA2(Y,CA)CU2O8-DELTA - THE FIRST CUPRATE SUPERCONDUCTOR CONTAINING A DOUBLE MERCURY OXYGEN LAYER

The first cuprate superconductor containing double (HgO)2 layers was synthesized using a high-pressure, high-temperature technique. The compound, with chemical formula Hg2Ba2Y1-xCaxCu2O 8-δ, contains a rock salt-type double HgO layer. The prototype compound Hg2Ba2YCu2O8-δ is an insulator. Superconductivity is induced in the system by partially replacing yttrium with calcium. © 1994

CRYSTAL-STRUCTURE OF THE DOUBLE-HG-LAYER COPPER-OXIDE SUPERCONDUCTOR (HG,PR)(2)BA-2(Y,CA)CU2O8-DELTA AS A FUNCTION OF DOPING

The crystal structure of the newly discovered double-Hg-layer copper oxide superconductor (Hg, Pr)2Ba2(Y, Ca)Cu2O8-δ was studied as a function of chemical doping using neutron and electron diffraction and high-resolution transmission electron microscopy (HREM). Rietveld refinements of the structural parameters from neutron powder diffraction data indicate that the oxygen site O3 on the mercury plane is highly defective, being both partially occupied and displaced from the high-symmetry position. The variable concentration of oxygen vacancies partially compensates for the cation doping and, together with the O3 displacement field, makes some of the Hg atoms acquire an unusual pyramidal coordination. HREM images confirm that the structure is of the '2212' type, with very few defects. In some grains, faint superstructure reflections were evidenced by electron diffraction, suggesting that both the oxygen vacancies and the O3 displacement field may order at least on a local scale. © 1995

Effects of Ca substitution on quasiacoustic sliding modes in Sr14−xCaxCu24O41

International audienceThe low-energy lattice dynamics of the quasiperiodic spin-ladder cuprate Sr 14−x Ca x Cu 24 O 41 are investigated using terahertz frequency synchrotron radiation. A high density of low-lying optical excitations is present in the 1-3 THz energy range, while at least two highly absorbing excitations stemming from long-wavelength acoustic oscillations of the incommensurate chain and ladder sublattices, are observed at subterahertz frequencies. The effects of Ca substitution on the subterahertz quasiacoustic sliding mode gaps is investigated using coherent synchrotron radiation. Analysis of the results suggest increasing substitution of Sr for Ca is accompanied by a transfer of spectral weight between sliding modes associated with different chain-ladder dynamics. The observation is consistent with a transfer of hole charges from the chains to the ladders and modification of the sublattice dimensions following Ca substitution. The results are discussed in context to the significance of lowlying vibrational dynamics and electron-phonon coupling in the superconducting state of certain quasiperiodic systems

First results on the study of reaction at q2 = - 4.2 (GeV/c)2

Here are reported the preliminary results of experiment PS170 at LEAR, at CERN, concerning e+e- production by p pbar annihilatio

MEASUREMENT OF THE PROTON ELECTROMAGNETIC FORM-FACTOR NEAR THRESHOLD IN THE TIME-LIKE REGION

The rate of the reaction pBARp --> e+ e- has been measured in liquid hydrogen for antiprotons at momentum values between 150 and 300 MeV/c. New precise values of the proton electromagnetic form factor in the time-like region have been obtained assuming \G(e)\ = \G(m)\ in the investigated region. The q2 dependence of the form factor close to threshold is found to have a steeper slope than indicated by existing calculations based on vector dominance models